In recent years many monoclonal antibodies immunologically reactive with tumor-associated antigens have been isolated (see, e.g., Papsidero, 1985, Sem. Surg. Oncol. 1: 171-81; Scholm et al. 1985 in Important Advances in Oncology, pages 170-192; Allum et al., 1986, Surg. Ann. 18: 41064; and Houghton et al., 1986, Semin. Oncol. 13: 165-179). The antigens bound by these monoclonal antibodies and characterization of the binding pattern of the monoclonal antibodies in most cases are incomplete and somewhat uncertain. Although a few have been completely characterized. Some of the more completely characterized monoclonal antibodies have been found to bind to a variety of tumor-associated antigens including glycoproteins, glycolipids and mucins (see, e.g., Fink et al., 1984, Prog. Clin. Pathol. 9: 121-133).
Murine monoclonal antibody L6 was raised against a human lung adenocarcinoma (Hellstrom et al., 1986, Cancer Res. 46: 3917-3923; Hellstrom et al., 1986, Proc. Natl. Acad. Sci. USA. 83: 7059-7063; U.S. Pat. No. 4,906,562; U.S. Pat. No. 4,935,495; and U.S. Pat. No. 5,091,177). In the initial effort to characterize the epitope to which monoclonal antibody L6 bound it was found that a glycolipid antigen, which in purified form had a terminal carbohydrate structure; EQU Ga1NAc.beta.1.fwdarw.4Ga1.beta.1.fwdarw.3Ga1NAc.beta.1.fwdarw.4Ga1.beta.1.f wdarw.R
as determined by solid phase ELISA and immunoblot of standard defined glycolipid structures separated by thin layer chromotography was recognized. Additionally, when L6 was used in radioimmune precipitation analysis of disrupted minor cells an unknown protein antigen of 20,000 dalton molecular weight was precipitated among others.
In later studies, L6 was found to stain intra-cytoplasmic material within the magnocellular component of the hypothalamoneuropophysial system. Further evaluation of this observation lead to the conclusion that L6 was binding to proneurophysin (Nilaver et al., 1990, Neuroendocrinology 51: 565-571 and Rosenbaum et al., 1990, Proc. Natl. Acad. Sci. USA 87: 9928-9932). A definitive characterization of the anitgen bound by L6 has been elusive and to date contradicting and so inconclusive that the antigen to which monoclonal antibody L6 binds is unknown.
Functionally, monoclonal antibody L6, an Ig G2a, has been found to mediate antibody-dependent cellular cytotoxicity (ADCC) with human mononuclear cells and complement-dependent cytotoxicity (CDC) with human complement. The antibody also has been found to localize to antigen-positive human tumor cells in nude mice (Lavie et al., 1989, Radiotherapy and Oncol. 15: 245-305) and inhbit their outgrowth (Hellstrom et al., 1986, Proc. Natl. Acad. Sci. USA 83: 7059-7063). Because the L6 antigen appears to be highly expressed on tumor cells and apparently expressed at a low level, or not at all, on most normal cell types monoclonal antibody L6 has been tried in several limited clinical studies. A phase I clinical study carried out in patients with recurrent cancer of either breast, colon, lung or ovary demonstrated that murine monoclonal antibody L6 was well tolerated and effectively localized to tumor in vivo and that one patient, with recurrent breast cancer on the chest wall, underwent complete, although temporary, remission (Goodman et al., 1990, J. Clinical Oncol. 8: 1083-1092). In other clinical studies, radiolabelled murine and chimeric (mouse-human) L6 antibodies were shown to deliver sufficient amounts of radioactivity to tumors to be of therapeutic interest (DeNardo et al., 1991, in Breast Epithelial Antigens, ed. R. L. Ceriani, Plenum Press, New York, pp. 227-232).
Although the clinical potential of monoclonal antibodies and various chimeras specific for the L6 antigen have been established, the definitive identification of the cell-surface tumor-associated antigen recognized by L6 has remained elusive. Definitive characterization of the antigen would provide insite not only into what the tumor-associated antigen may be, but also supply a reagent for use in enhancing a more specific productive immune response to a tumor.